Investment casting often utilizes cores to produce internal channels inside cast metals. A molten metal or alloy is poured into a mold containing a core. After the metal solidifies, the core is removed to leave behind the internal channels. The architecture of the internal channels is determined by the features of the core.
Cores formed through injection molding and other conventional processes can produce simple hollow channel architectures. However in some applications, such as cast blades for gas turbines, more complex channel geometries are desirable due to their improved blade performance, where air is blown through the hollow channels of the cast blade for cooling. Improved blade cooling performance can take the form of reduced cooling air flow, which allows for increased utilization of air for combustion and thus increases engine thrust. Higher blade cooling performance allows for an increase in combustor operating temperature and improved thermodynamic efficiency, resulting in better specific fuel consumption, while still maintaining turbine blade component temperatures within an acceptable range for durability. Especially useful channel geometries for turbine blade cooling circuits are described in, for instance, U.S. Pat. No. 5,660,524, U.S. Pat. No. 6,036,441, U.S. Pat. No. 6,168,381, U.S. Pat. No. 6,595,748 and U.S. Pat. No. 6,832,889. A major limitation to commercial implementation of these representative cooling circuits in turbine blades is the inability to produce the necessary ceramic cores as 1-piece articles by conventional molding techniques. Examination of the cooling circuit geometries of interest shows that there exists no single parting line allowing the construction of separable mold halves to enable removal of a molded part without destroying part of the formed structure. Accordingly, production of cores to produce such sophisticated cooling circuits requires elaborate multi-step processes where the geometry is broken up into several moldable sections, each with its own separate mold tooling. The individual molded sections are then assembled after molding and firing, with a concomitant reduction in core yield for precision investment casting, due to the loss of registry between the parts. The multi-piece process is also much more expensive due to the capital cost of multiple mold tools, the hand work needed for core finishing and assembly, and the further reduction in net casting yield, due to the poorer dimensional tolerances and mechanical stability during casting of the multi-piece core assembly.
One way to produce cores with both simple and complex channel architectures is with disposable core dies (DCD) described in, for instance, U.S. Pat. No. 7,487,819. The core is formed by injecting a slurry containing ceramic particles and an organic binder into a disposable core die. The slurry is then cured and then fired to produce a solidified ceramic core. The disposable core die can be removed before or during or after the core firing process, for instance by a chemical, thermal or mechanical process.
Ceramic core materials used in the investment casting industry are often made predominately of silica (SiO2). Silica is a commonly used core material in investment casting because of its low coefficient of thermal expansion, high-temperature dimensional stability, and its ease of removal from the casting. Articles made by investment casting are cast metal or metal alloys. In some instances, this metal may react with the conventional silica-based ceramic core. Therefore, the use of silica-containing core materials for casting of reactive metals is known to be problematic, as silica may react with certain metals during the casting process.
Yttrium addition to an alloy is one approach for improving the oxidation resistance of nickel-based superalloys at the service temperature of turbine airfoils. However, yttrium can react with silica during casting, leading to depletion of the yttrium in the alloy and introducing components into the alloy that debit the mechanical properties. This is a major limitation in using silica cores for the casting of these (reactive) nickel-based superalloys.
Alumina and yttria materials have been used in casting to reduce or eliminate this reactivity problem. Alumina, for instance, is less reactive than silica. However, alumina is harder to process than silica materials with respect to higher temperatures required for processing. This introduces problems with dimensional tolerances because of the higher coefficient of thermal expansion relative to silica. Alumina cores can also require more extreme leaching conditions for their removal after casting because of their lower solubility and/or leach rates. These constraints apply equally to ceramic cores formed through conventional methods such as injection molding, as well as those formed by the DCD process.
Alumina core compositions of the prior art useful for casting reactive alloys, such as U.S. Pat. No. 4,837,187 and U.S. Pat. No. 5,409,871 and U.S. Pat. No. 5,580,837, are known, and consist of alumina and other ceramic additives in a thermoplastic organic polymer binder. The polymer binders are solids at room temperature, and must be mixed at elevated temperatures in the molten state. These compositions are subsequently used in high pressure resin-transfer molding processes, which must also occur at elevated temperatures. The U.S. Pat. No. 4,837,187 employs an ethylene-vinyl acetate polymer and wax mixture that is mixed and molded from 80-125° C. and at 200-1500 psig pressure. The U.S. Pat. Nos. 5,409,871 and 5,580,837 disclose a hydrophilic and a hydrophobic binder-ceramic powder mixture which must be compatibilized by mixing and injecting at 200° C., as described in U.S. Pat. No. 5,332,537. This mixture has a comparatively low viscosity from 5 to 300 Pa-sec, but relies on being heated to this high temperature in order to achieve this viscosity and maintain mixture homogeneity during use. It is not only more costly to run either such process with the energy expenditure to maintain the core mixture and injection process equipment at a temperature greatly elevated above the ambient, but both are also incompatible with the use of fugitive organic polymeric core dies that lose their rigidity above about 60° C. None of these cases teach the formation or use of a low-reactivity or non-reactive core compositions with properties compatible with the DCD process, which are ideally conducted at or about ambient room temperature and at pressures <100 psig.
While the use of Y2O3 coatings on the opposing side of the metal in shells has been practiced for Y-containing reactive alloys, there does not appear to be any well-practiced art for pure Y2O3 cores. Further, yttria is not an ideal material for a non-reactive core. The cost of the rare earth oxide versus alumina or silica is much higher (>10×). Yttria has lower core strength due to the lower bulk modulus as compared to alumina. Yttria also shows poor leaching in conventional pressurized autoclaves with strong aqueous caustic solutions. Additionally, yttria shows a high thermal expansion, with a CTE of 7 ppm (as does alumina).
Bochiechio (US 2014/0182809) teaches the use of mullite- and metallic-containing cores for investment casting. US 2014/0182809 is focused on the use of these compositions to closely match the coefficient of thermal expansion of the ceramic material to a refractory metal component, and does not teach the casting of reactive alloys with these compositions. To the contrary, its allowance of up to 60 wt % silica indicates to one of ordinary skill in the art that the castings of reactive alloys are not envisioned. The present disclosure uses cores that are suitable for casting of reactive alloys, in stark contrast to Bochiechio.
Therefore, there is a need for a low reactivity material system compatible with DCD processing for producing cores that can generate cast articles with complex internal channel architectures made using reactive metals and alloys.